Preliminary studies have indicated that the centromeres in mitotic cells of some species divide and separate into daughter centromeres in a pre-determined, non-random and sequential fashion. In man those chromosomes which separate last at the centromere to release the chromatids for anaphase movements appear to be involved in aneusomy more often than those whose centromeres separate early in the sequence. The study proposes, using whole animal and plant species as well as tissue cultures involving 8 species in all to make chromosome preparations, select cells which have from one to all but last chromosome having separated at the centromere, and determining the sequence in which the daughter centromeres separate. A comparison between mitotic cells with those in second meiotic division is expected to produce parallel data. Efforts will be made to determine if polymorphism exists for this trait among some ethnic groups viz, Caucasians, Negros, Asian Indians, American Indians and Orientals. The proposal also calls for finding any relationship between position of a centromere in separation sequence and the amount of C-chromatin (paracentromeric constitutive heterochromatin) using chromosomes in mouse, tobacco mouse, Microtus and man showing differential quantities of C-chromatin between homologues or analogues. The unequal distribution of mitotic chromosomes by some chemicals raises the interesting possibility that such agents induce aneusomy by disturbing the sequence of centromere separation. This concept shall be tested with the hope that these studies will lead to the development of a new rapid test system for induced non-disjunction by environmental mutagens.